1. Field of the Invention
This invention relates to the enhancement of heat transfer in a multi-tubular reformer intended for the production of a reformed gas to be used in a fuel cell. More particularly, this invention, in the production of a reformed gas to be used in a fuel cell by using a multi-tubular reformer provided with a plurality of bayonet type double-wall catalyst tubes in a pressure vessel, relates to a method of exalting the utilization efficiency of the heat of the flue gas and ensuring uniform heating of the catalyst tubes by preventing the shell wall effect in thermal radiation and also preventing channelling of the flue gas in the shell side and a reformer employing the method mentioned above.
2. Related Background Art
The electric power generation with a fuel cell has been attracting attention as a method of electric power generation which enjoys high efficiency and low polution to the environment. The fuel cell operates on the principle that a direct current is extracted through an external circuit connected to an anode (hydrogen electrode) and a cathode (oxygen electrode) by feeding continuously hydrogen and oxygen respectively to the anode and the cathode with an electrolyte disposed between the two electrodes.
The fuel cell is known in various types such as the alkali type (AFC), the phosphoric acid type (PAFC), and the molten carbonate type (MCFC), which depend on the kind of electrolyte to be used. These types are alike in feeding hydrogen (pure hydrogen or crude hydrogen) to the anode but are unlike in tolerating or not tolerating carbon dioxide or carbon monoxide in the hydrogen gas. The alkali type does not tolerate carbon dioxide or carbon monoxide in the hydrogen gas because carbon dioxide impairs the function of potassium hydroxide as an electrolyte and carbon monoxide poisons platinum being used as a catalyst and they both go to degrade the performance of the fuel cell. Thus, this type is at a disadvantage in necessitating use of substantially pure hydrogen as a fuel gas. The phosphoric acid type tolerates carbon dioxide but does not tolerate carbon monoxide because the latter poisons platinum being used as a catalyst. Thus, this type permits use of crude hydrogen on the condition that the carbon monoxide in the crude hydrogen be caused to react with steam in a carbon monoxide converter and converted into carbon dioxide and hydrogen. The molten carbonate type does not use a platinum catalyst and, therefore, tolerates both carbon dioxide and carbon monoxide in the hydrogen gas and, moreover, enjoys the advantage that the carbon monoxide reacts with the water generated at the anode to generate hydrogen which can be utilized as the fuel. Since the molten carbonate type has many other economic merits, it offers a promising prospect as an alternative to the thermal electric power generation of the medium to small scale. Besides the types mentioned above, the fuel cell is known in the solid electrolyte type (SOFC), the polymer type (PFC) and so forth. Though these types have characteristics of their own, they are alike those mentioned above in feeding hydrogen to the anode.
The crude hydrogen which is fed to the anode in the phosphoric acid type or the molten carbonate type fuel cell is generally produced by using hydrocarbon, such as natural gas (having methane as a main component) as a raw material and causing it to react with steam. This reaction is thought to proceed as represented by the following formula (1) or (2). EQU CH.sub.4 +H.sub.2 O.fwdarw.CO+3H.sub.2 ( 1) EQU CH.sub.4 +2H.sub.2 O.fwdarw.CO.sub.2 +4H.sub.2 ( 2)
The reaction is endothermic in nature and is generally carried out in the presence of a catalyst at a temperature in the range of from 600.degree. C. to 1000.degree. C. The reformer is an apparatus to be used for implementing this reaction. It is generally so constructed as to pass the feed gas (natural gas and steam) through reaction tubes loaded with a catalyst and heat externally the reaction tubes by means of a burner or a hot fluid produced by catalytic combustion.
The reformer provided with bayonet type double-wall catalyst tubes, among other types, has been in popular use to date. A typical structure of the reformer of this type is illustrated schematically in FIG. 1. In FIG. 1, natural gas and steam as raw materials are introduced into a pressure vessel 1 via a feed gas inlet 2 disposed at one end (the top part in the diagram) of the pressure vessel and caused to flow through the clearances between inner tubes 3 and outer tubes 4 which jointly constitute bayonet type double-wall catalyst tubes. These clearances are loaded with a reforming catalyst 5. While the feed gas is flowing through the catalyst layer, the natural gas and the steam react with each other to produce a hydrogen-containing reformed gas. Then, the produced reformed gas is advanced inside the inner tubes 3 and fed through a manifold 6 to the fuel cell (not shown) via a reformed gas outlet 7. The pressure vessel 1 is provided at the other end (the bottom part in the diagram) with a flue gas feed part, which comprises a flue gas feed chamber 8 and a flue gas inlet 9 as illustrated in FIG. 1, for example. The flue gas is made to flow along the outer side (the shell side) of the outer tubes 4 and meanwhile heats the catalyst 5 externally and then is discharged through a flue gas outlet 11. The inner tubes 3 are supported by the manifold 6 and the outer tubes 4 are supported by a tube sheet 12. Since these two groups of tubes are not fixed relative to each other, no stress due to the difference in thermal expansion between the outer tubes and the inner tubes (generally the outer tubes are elevated to a higher temperature) is generated. This very fact constitutes one of the merits found in the adoption of the bayonet type catalyst tubes in the fuel cell reformer which is required to tolerate frequent start-ups and shut-downs and frequent load changes. As a fuel gas for the combustor, generally the exhaust gas from the anode of the fuel cell (which contains unreacted hydrogen because the hydrogen fed to the fuel cell is not wholly utilized) is utilized. This exhaust gas is not easily burnt by itself when it has a small hydrogen content and therefore a low calorific value. Thus, the catalytic combustion is used in the place of the ordinary burner combustion.
As described above, the heat necessary for the reforming reaction is supplied by the flue gas. The flue gas at an elevated temperature (1000.degree. C. to 1500.degree. C.) is made to flow in the shell side of the interior of the pressure vessel and meanwhile heat externally the outer tubes of the bayonet type catalyst tubes. Since the efficiency of heat transfer between the flue gas and the catalyst tubes generally is not very high, various measures have been adopted to date for enhancing the efficiency of heat transfer. For example, the practice of arranging sleeves, orifice baffles, or wire nets near the shell sides in the basal parts of the catalyst tubes (the parts near to the tube sheet and far from the flue gas inlet part) has been made. Though these conventional measures contribute to improving the aforementioned efficiency of heat transfer to a certain extent, the contribution does not deserve to be called fully satisfactory. Further, these measures barely go the length of improving the efficiency of heat transfer in the basal parts of the catalyst tubes after all because they cannot be employed from structural or material reason in the leading parts of the catalyst tubes (the parts far from the tube sheet and near to the flue gas inlet part).
When the capacity of a reformer reaches the scale of multi-megawatts, the number of catalyst tubes falls into the range of from 30 to 60. Thus, the reformer is likely to have uneven operating temperatures among the catalyst tubes. The differences in the operating temperature frequently reach as much as .+-.100.degree. C. in conventional reformers. The catalyst tubes of higher temperatures suffer from lowered life span and sintering of the reforming catalyst owing to overheating and those of lower temperatures suffer from impaired efficiency of electric power generation owing to a lowered conversion of the raw materials. The following three causes are conceivable for the uneven heating of the catalyst tubes. Firstly, since the heat radiated from the shell wall which has a large heat capacity and a high thermal radiation emissivity elevates excessively the temperatures of the catalyst tubes located particularly in the outer peripheral part of the interior of the pressure vessel (the so-called wall effect) and these catalyst tubes themselves which are so heated to the elevated temperatures radiate heat, the heat balances of the individual catalyst tubes are disturbed by the wall effect or shape factor in thermal radiation between the adjacent catalyst tubes. Secondly, since the flue gas flowing in the shell side frequently generates channelling, those catalyst tubes which are located in the area of a large gas flow rate are heated to higher temperatures. Thirdly, since the feed stock gas flowing in the process side (the catalyst layer) often fails to flow uniformly through the individual catalyst tubes, the amount of heat absorbed by the reforming reaction (endothermic) gets uneven among the individual catalyst tubes, resulting in uneven operating temperatures.